Lining structural monitoring in the new underground service of Naples (Italy)

A new underground stretch connecting two existing lines of the urban railway underground system was built in Naples. The tunnel was excavated by NATM through unsaturated pyroclastic silty sand at relatively large depths, ranging between thirty to eighty metres. The geometry of the section is rather typical with a top part shaped as half a circle and a rectangular bottom. The tunnel was conventionally built with a staged construction: after the full face excavation, a temporary lining composed by horse-shoe steel ribs and sprayed concrete was put in place. Two steel ribs, spaced about one hundred metres apart and located about sixty metres below the ground surface, were instrumented with twenty-four vibrating wire strain gauges, installed at six symmetric locations. Nearly two years of measurements were collected. In the paper the measured strains and the back-calculated internal forces are reported. The results of FE 3D and 2D back-analyses are also shown allowing to throw a light on the observed behaviour.